1. Field of the Invention
The present invention relates to an image forming apparatus having a convey belt for bearing and conveying a recording material. More particularly, it relates to an image forming apparatus which is suitable to be applied to a color copying machine, a laser beam printer and the like, in which images formed on a plurality of image bearing members are transferred onto a recording material born by a convey belt in a superimposed fashion.
2. Related Background Art
In the past, for example, there has been proposed a image forming apparatus in which a plurality of image forming units each having an image bearing members (such as a photosensitive drum) are disposed side by side and images formed on the image bearing members are successively transferred onto a recording material (recording sheet) born on a transfer belt, thereby forming a multi-color image. Such an image forming apparatus will now be briefly explained with reference to FIG. 9.
The image forming apparatus (color laser printer utilizing a Carlson electrophotographic method for forming a latent image) comprises a transfer material convey belt (convey means) 7 for conveying a transfer material (transfer sheet) 9, four image forming units Pa, Pb, Pc and Pd disposed side by side along a shifting direction of the transfer material convey belt 7, and a fixing means 10. The image forming units Pa to Pd include photosensitive drums 1a, 1b, 1c and 1d, charge means 2a, 2b, 2c and 2d, laser beam scanners 3a, 3b, 3c and 3d, developing means 4a, 4b, 4c and 4d, transfer means 5a, 5b, 5c and 5d, and cleaning means 6a, 6b, 6c and 6d, which means 2a to 2d, 3ato 3d, 4a to 4d, 5a to 5d and 6a to 6d are disposed around the respective photosensitive drums 1a to 1d.
In the first image forming unit Pa, the photosensitive drum 1a uniformly charged by the charge means 2a is exposed by a laser beam from the laser beam scanner 3a, thereby forming a latent image for a yellow color component of an original image on the photosensitive drum 1a. Thereafter, the latent image is visualized by the developing means 4a with yellow toner to form a toner image. Then, the toner image is transferred onto the transfer sheet 9 on the transfer material convey belt 7, by means of the transfer means 5a.
On the other hand, while the yellow tone image is being transferred onto the transfer sheet 9 by the transfer means 5a, in the second image forming unit Pb, a latent image for a magenta color component of the original image is formed on the photosensitive drum 1 by the laser beam scanner 3b, and, then, the latent image is developed by the developing means 4b with magenta toner to form a magenta toner image. When the transfer sheet 9 to which the toner image was transferred in the first image forming unit Pa reaches the transfer means 5b of the second image forming unit Pb, the magenta toner image is transferred onto a predetermined position on the transfer sheet 9.
Similarly, a cyan toner image and a black toner images are successively transferred onto the same transfer sheet 9. After four color toner images are transferred to the transfer sheet 9 in a superimposed fashion, the transfer sheet 9 is sent to the fixing means 10, where the toner images are fixed to the transfer sheet 9, thereby forming a full-color image. After the transferring operation, residual toner is removed from each of the photosensitive drums 1a to 1d by the respective cleaning means 6a to 6d for preparing for the next latent image formation.
By the way, in the color image forming apparatus having the above-mentioned construction, since the plurality of image forming units Pa to Pd are provided for different color toners, the image forming operation can be effected at a high speed, and, since the transferring processes of the image forming units Pa to Pd are effected along a straight path, a thick sheet or a transparent sheet can be used as the transfer sheet. To the contrary, there arises a problem that the registration between the different color toner images formed in the respective image forming units Pa to Pd and the transfer sheet 9 must be performed strictly. If the registration between the different color toner images with respect to the transfer sheet 9 is not performed strictly to cause the deviation (referred to as "regist-shift" hereinafter) between the images on the transfer sheet, a poor multi-color image having color deviation and/or change in color phase will be obtained.
The regist-shift is caused by the fact that the transfer material convey belt 7 is not moved straightly but is shifted toward one side out of predetermined areas of a drive roller 11, an idler roller 12 and an adjustment roller 13, 14 shown in FIG. 9 or is moved along a serpentine route. Such regist-shift becomes one of factors for the color deviation.
Conventionally, in the color image forming apparatus having the above-mentioned construction, in order to prevent the one-side shift of the belt, for example, the following methods have been proposed:
(a) A method for providing flanges on both longitudinal ends of the rollers such as the drive roller 11, idler roller 12 and the like. PA1 (b) A method for providing so-called "crowns" on the rollers such as the drive roller 11, idler roller 12 and the like so that a diameter of a central portion of each roller becomes greater than diameter of longitudinal ends of the roller. PA1 (c) A method for providing a sprocket on one longitudinal end of each roller and forming a series of perforations in one edge portion of the transfer material convey belt with a predetermined interval equal to a pitch between teeth of the sprocket.
However, in the above method (a), although a simple construction can be obtained, since the transfer material convey belt 7 is positioned by urging the longitudinal edges of the belt against the flanges, the longitudinal edges of the belt are always subjected to thrust forces, with the result that the longitudinal edges of the belt are easily deformed or damaged. Thus, it is difficult to maintain the stable movement of the belt for a long time. In particular, in a transfer material convey belt made of organic material, it is desirable that a thickness of the belt is reduced as much as possible in order to improve the transferring efficiency. When the belt having the small thickness is used, since the belt edges have no strength, the belt can easily be damaged or destroyed, and, thus, cannot be used for a long time.
Such inconvenience will occur in the proposals disclosed in the Japanese Patent Publication Nos. 57-60347 (1982) and 56-156544 (1981) which utilize methods similar to the above method (a).
The above method (b) is a correction method which has been widely used. However, since the endless transfer material convey belt is flexed by the crowns of the rollers to generate internal stress, thereby preventing the one-side shift of the belt, the belt must be made of material having sufficient elasticity. In addition, since so-called permanent deformation of the belt due to creep strain must be prevented while utilizing the elastic deformation (strain), for example, when the belt is formed from rubber, not only rubber material having low hardness to provide adequate elasticity must be utilized, but also the thickness of the belt must be increased sufficient to provide predetermined mechanical strength. Thus, when the above method (b) is adopted to the transfer material convey belt 7, since the thickness of the transfer material convey belt 7 must be increased, the transfer current must also be increased. This is not preferable.
In the above method (c), as is in the above method (a), since the belt must have the required strength (particularly, anti-break strength) and the pitch accuracy of the sprocket teeth and the perforations greatly affects an influence upon the moving speed accuracy of the belt, high accurate moving speed control is required (for the transfer material convey belt 7). This is also not preferable.
In order to overcome the drawbacks of the above methods (a), (b) and (c), there was provided a method in which at least one of rollers among the rollers having axes perpendicular to a shifting direction of the transfer material convey belt 7 has a rotation axis having a variable angle with respect to the shifting direction of the transfer material convey belt 7.
In this method, a detection means for detecting one-side shift of the belt is provided, and, by changing the angle of the rotation axis of the roller on the basis of an output signal from the detection means, the one-side shift of the belt is corrected. This method is particularly effective to the belt having low elasticity and the effect of this method is not influenced upon the thickness of the belt.
However, according to the test results conducted by the Inventors, as shown in FIG. 9, in the image forming apparatus in which the plurality of photosensitive members are disposed side by side and the toner images formed on the photosensitive members are successively transferred onto the transfer sheet born on the transfer material convey belt, thereby forming a multi-color image, since the image is transferred onto the transfer sheet while the images are always being formed on the plural photosensitive members, after the one-side shift (offset) of the belt is detected by the detection means, if the moving direction of the belt is abruptly changed, it was found that the positions on which the different color toner images are to be transferred are also changed, with the result that the color deviation is caused along the direction perpendicular to the shifting direction of the belt, thereby worsening the image quality.
The present invention aims to eliminate such inconvenience.